Answer:
HC₉H₇O₄(aq) + H₂O(l) ⇄ C₉H₇O₄⁻(aq) + H₃O⁺(aq)
Step-by-step explanation:
A weak acid as aspirin, HC₉H₇O₄, is in equilibrium with water as follows:
HC₉H₇O₄(aq) + H₂O(l) ⇄ C₉H₇O₄⁻(aq) + H₃O⁺(aq)
This is the chemical equation that describes the process.
As you can see, not all aspirin is dissolved, just a part that is converted in its conjugate base (A weak acid doesn't dissociate completely!).
Also, the pH is decreasing because the hydronium ion (H₃O⁺) is produced in the equilibrium (Remember, if you have an increasing in H₃O⁺ concentration the pH will decreases and vice versa).